Knowing the exact value of the radiation temperature is essential for the calibration process. It can be measured by either using a contact thermometer (in combination with the determination of the emissivity) or a transfer standard infrared thermometer. This value can then be used to determine the device constant for an initial calibration of the infrared sensors. In order to conduct a post-calibration by customers or local calibration facilities, the calibration temperature should be near the temperatures which occur at the respective applications.

Optris makes use of a transfer standard radiation thermometer CTlaser-PTB (see figure) to measure the radiation temperature of a reference source. The CTlaser- PTB is based on the IR thermometer optris CTlaser. The CTlaser-PTB needs to be traceable to the international temperature scale from 1990 (ITS-90). Thus, it is calibrated by the PTB (German national metrology institute) on a regular basis. ITS-90 is a very good approximation of thermodynamic temperature. It is based on 17 well-reproducible fixed values such as melting points of highly pure metals. Within the framework of ITS-90 the CTlaser-PTB is compared to national temperature standards from the PTB. This comparison within a closed chain of comparative measurements with a known uncertainty in measurement takes place on a regular basis.

Based on the CTlaser-PTB, Optris produces the CTlaser- DCI as a high-precision reference IR thermometer for its customers. The DCI units are produced with pre-selected components supporting high stability of measurement. In combination with a dedicated calibration at several calibration points the CTlaser-DCI achieves a higher accuracy than units from series production.

The optics of an IR thermometer is described by the distance-to-spot-ratio (D:S). Depending on the quality of the optics a certain amount of radiation is also received from sources outside the specified measurement spot. The maximum value here equals the radiation emitted by a hemispheric radiant source. The respective signal change in correlation with a resize of the radiation source is described by the Size-of-source effect (SSE).

As a result of this correlation all manufacturers of IR thermometers use accurately defined geometries for the calibration of their units; meaning depending on the aperture of the radiation source (A) a distance (a) between the IR thermometer and the reference source is defined. Thus, the value specified in datasheets and technical documentation as a measurement field is, in general, a certain defined percentage of this radiation maximum – values of 90 % or 95 % are common.